Recent growth in demand for broadband wireless services enables rapid deployment of innovative, cost-effective, and interoperable multi-vendor broadband wireless access products, providing alternatives to wireline broadband access for applications such as telephony, personal communications systems (PCS) and high definition television (HDTV). At the same time, broadband wireless access has been extended from fixed to mobile subscriber stations, for example at vehicular speed. Though the demand for these services is growing, the channel bandwidth over which the data may be delivered is limited. Therefore, it is desirable to deliver data at high speeds over this limited bandwidth in an efficient, as well as cost effective, manner.
In the ever-continuing effort to increase data rates and capacity of wireless networks, communication technologies evolve. An encouraging solution for the next generation broadband wireless access delivering high speed data over a channel is by using Orthogonal Frequency Division Multiplexing (OFDM). The high-speed data signals are divided into tens or hundreds of lower speed signals that are transmitted in parallel over respective frequencies within a radio frequency (RF) signal that are known as subcarrier frequencies (“subcarriers”). The frequency spectra of the subcarriers may overlap so that the spacing between them is minimized The subcarriers are also orthogonal to each other so that they are statistically independent and do not create crosstalk or otherwise interfere with each other. When all of the allocated spectrum can be used by all base stations, the channel bandwidth is used much more efficiently than in conventional single carrier transmission schemes such as AM/FM (amplitude or frequency modulation), in which only one signal at a time is sent using only one radio frequency, or frequency division multiplexing (FDM), in which portions of the channel bandwidth are not used so that the subcarrier frequencies are separated and isolated to avoid inter-carrier interference (ICI).
In OFDM, each block of data is converted into parallel form and mapped into each subcarrier as frequency domain symbols. To get time domain signals for transmission, an inverse discrete Fourier transform or its fast version, IFFT, is applied to the symbols. The symbol duration is much longer than the length of the channel impulse response so that inter-symbol interference is avoided by inserting a cyclic prefix or a predefined value for each OFDM symbol. Thus, OFDM is much less susceptible to data loss caused by multipath fading than other known techniques for data transmission. Also, the coding of data onto the OFDM subcarriers takes advantage of frequency diversity to mitigate loss from frequency-selective fading when forward error correction (FEC) is applied.
Another approach to providing more efficient use of the channel bandwidth is to transmit the data using a base station having multiple antennas and then receive the transmitted data using a remote station having multiple receiving antennas, referred to as Multiple Input-Multiple Output (MIMO). The data may be transmitted such there is spatial diversity between the signals transmitted by the respective antennas, thereby increasing the data capacity by increasing the number of antennas. Alternatively, the data is transmitted such that there is temporal diversity between the signals transmitted by the respective antennas, thereby reducing signal fading.
Wireless communication systems divide areas of coverage into cells, each of which is served by a base station. A subscriber station will continuously monitor the signal strengths of the servicing base station for the current cell as well as for adjacent cells. The subscriber station will send the signal strength information to the network. As the subscriber station moves toward the edge of the current cell, the servicing base station will determine that the subscriber station's signal strength is diminishing, while an adjacent base station will determine the signal strength is increasing. The two base stations coordinate with each other through the network, and when the signal strength of the adjacent base station surpasses that of the current base station, control of the communications is switched to the adjacent base station from the current base station. The switching of control from one base station to another is referred to as a handoff.
A hard handoff is a handoff that completely and instantaneously transitions from a first station to a second base station. Hard handoffs have proven problematic and often result in dropped calls. Wireless systems may incorporate a soft handoff, wherein when the subscriber station moves from a first to a second cell, the handoff process happens in multiple steps. First, the subscriber station recognizes the viability of the second base station, and the network allows both the current and adjacent base stations to carry the call. As the subscriber station move closer to the second base station and away from the first base station, the signal strength from the first base station will eventually drop below a useful level. The subscriber station will then inform the network, which will instruct the first base station to drop the call and let the second base station continue servicing the call. Accordingly, a soft handoff is characterized by commencing communications with a new base station before terminating communications with the old base station.
In orthogonal frequency division multiplexing access (OFDMA) systems, multiple users are allowed to transmit simultaneously on the different subcarriers per OFDM symbol. In an OFDMA/TDMA embodiment, for example, the OFDM symbols are allocated by time division multiplexing access (TDMA) method in the time domain, and the subcarriers within an OFDM symbols are divided in frequency domain into subsets of subcarriers. In other embodiment, to average inter-cell interference, different cells may use, for example, different permutations to generate subchannel.
It is therefore desirable to provide soft handoff to broadband wireless access system employing OFDMA. Because different spreading code masking is not available in OFDM transmission, the destructive interferences between base stations transmitting the same signal can cause significant degradation of performance.
It is further desirable to define a soft handoff zone with same permutation wherein the base stations provide RF combining, interference avoidance, soft combining, or selection combination in the handoff area. It is further desirable to use multi-input, multi-output (MIMO) method in a soft handoff of an OFDMA system. This MIMO method may apply to a plurality of base stations, each of the base stations has one, or more than one antennas.
Accordingly, there is a need for an efficient soft handoff technique for OFDMA systems as well as a need to increase data rates and reduce interference at cell borders. It is further desirable to provide soft handoff technique to a MIMO OFDMA system.